This invention relates to alarm circuits and, more particularly, to audio-visual alarms.
Persons with normal sight and hearing are more likely to notice a signal that is both auditory and visual. For example, when the background light or sound is very intense, a visual or auditory signal, respectively, is less readily detected. Since it is not always possible to know in advance whether the background light or sound will be sufficiently low to permit reliable detection of a signal, providing both types of signals together will give people the best opportunity to receive the warning.
According to standard 72A of the National Fire Protection Association (NFPA), the signal for evacuation of a building is three short audible bursts. However, people who are deaf will not be alerted to this or other alarms. If a visual alarm is substituted for the audio alarm, blind people will not be warned. Therefore, it would be advantageous for this segment of the population if an audio-visual alarm were provided. Further, the audio and visual signals according to Standard 72A should be synchronized so as to reduce confusion among the people who can both see and hear.
Some audio-visual signaling devices, such as that in U.S. Pat. No. 2,696,598 to Lozowski, supply the power for an audio signal, such as a horn, and a visual alarm, e.g. a lamp, over separate circuits, thus requiring complicated mechanisms for operating one of the signals in response to the activation of the other.
When a bell system is used as an audio alarm, a coil for activating the clapper is placed in series with the clapper contacts and a power source. When the power is turned on the contacts open due to the field set up by the coil. This contact opening interrupts the current, thereby causing the field to collapse and the contacts to close again. This repeats at the natural frequency of the unit. According to U.S. Pat. No. 3,810,149 to Miller et al., the collapsing field produced by the bell coil can be used to generate a voltage that operates a neon signal light connected across the coil, thus providing both audio and visual signals. A neon light, however, does not provide the intensity of signal that is desirable to overcome background lighting. U.S. Pat. No. 4,101,880 which issued to Rein Haus, the present inventor, and is assigned to the assignee of the present invention, also discloses a bell system operated by placing the bell coil, clapper contacts and a voltage source in series, as in the Miller et al. patent. However, the field from the coil is used to charge a storage capacitor through a diode. When the voltage across the capacitor has reached a critical voltage, a Xenon flashtube in parallel with the capacitor operates and the bell is momentarily silenced. Thus a strong light flash is produced along with a pulsating bell signal that is synchronized to it. Nevertheless, it would be advantageous if a single stroke audible alarm could be operated simultaneously with a flashtube by means of a simple, inexpensive and energy efficient circuit.